The present invention relates to a current limiting system and a current limiting method for fuel cells, for limiting electric current generated by a fuel cell stack in accordance with an electricity-generation condition of the fuel cell stack.
Fuel cell includes a solid polymer electrolyte membrane sandwiched between a hydrogen pole at one side of the membrane and an oxygen pole at the other side of the membrane to provide a membrane electrode assembly (MEA), which is further sandwiched between conductive separators to form a single cell. Several tens or several hundreds of single cells are stacked to form a fuel cell stack. In order to generate electricity stably and constantly in this kind of fuel cell stack, the fuel cell stack has to be driven under a certain constant temperature, for example, at 80° C., and as disclosed in Japanese Laid-open Patent Application No. 7-272736 (paragraphs [0019] and [0020]), all the cell voltages are monitored and current limitation is performed in consideration of the lowest cell voltage such that the output voltage (cell voltage) of any one of these single cells does not become lower than a predetermined threshold value. In the case where the temperature of the fuel cell stack is lower than a predetermined temperature at the time of operation of the fuel cell stack, or in the case where the solid polymer electrolyte membrane is not sufficiently humidified to a predetermined humidified state and the ion conductivity has been deteriorated, the electricity generating efficiency deteriorates and the cell voltage of each single cell often lowers accordingly. While the cell voltage has been lowering, if generated electric current is taken out as much as in the case of the normal state where a decrease in cell voltage has not been occurred, the fuel cell stack becomes unstable, resulting a further decrease in the cell voltage or a deterioration of the solid polymer electrolyte membrane which ends in deteriorated durability of the fuel cell stack.
As to the problem regarding durability of the fuel cell stack, it may be solved by the technique disclosed in the Japanese Laid-open Patent Application No. 7-272736, in which the current limit value is set in accordance with the lowest cell voltage to limit the generated current. However, in this technique, notwithstanding the amount of generated electricity taken out from the fuel cell stack, the current control is carried out by changing the current limit value as soon as the lowest cell voltage varies. Therefore, even if the amount of generated current is so small, which is not required to limit the generated current, the current limit value is frequently renewed. Such unnecessary operations often make the control of the fuel cell stack unstable.
In view of the above, the present invention seeks to provide a current limiting system and a current limiting method for fuel cells, which enable a control in accordance with cell voltages of the fuel cell stack, and which contribute to a stable control for generating electricity at the fuel cell stack while eliminating unnecessary operations.